Method and installation for the production of lime water from deionised water

ABSTRACT

A method of producing lime water comprising deionising dilution water by membrane filtration, distillation or an ion exchange resin and adding silicate ions to the deionised water. Thereafter, the deionised water treated with the silicate ions is mixed with milk of lime to form lime water. In one embodiment, the milk of lime and dilution water are mixed in a saturator where the silicate ions are introduced.

The invention relates to the domain of water treatment.

More precisely, the invention relates to the domain of production of lime water for use in remineralising water with low mineral content and more precisely water that has been subjected to a deionisation step either by membrane filtration (of the reverse osmosis, nanofiltration type), or by distillation, or by passage on an ion exchange resin.

This water may be slightly or not at all mineralised, and has the disadvantage of being corrosive and therefore inducing degradation to installations used for transport or treatment of the water.

Furthermore, this water has a high equilibrium pH value, in the sense of calcocarbonic equilibrium of the water. This high value is incompatible with good efficiency of some disinfectants such as chlorine. Therefore, the water should be remineralised in order to reduce the equilibrium pH value.

In the remineralisation method, it is known that a base, and particularly limewater, can be added to the water.

Special devices called “saturators” have to be used due to the low solubility of lime in water, so as to obtain a limewater with constant titre enabling precise dosage.

Ideally, the objective is thus to produce quality lime water using such saturators, with a titer in practice of about 220° F. TAC (total alkali strength) at a temperature of 20° C., namely 1.65 g/liter of Ca(OH)₂ with a turbidity less than 10 NTU. Due to its clarity, this type of limewater in the form of a saturated solution may be mixed with water to be treated to efficiently remineralise it using gaseous CO₂.

Conventionally, saturators receive firstly a quantity of lime in the form of milk of lime (of the order of 20 to 100 g/liter) injected into the saturator base or into a mixing chamber internal to or external to the saturator, and secondly dilution water (water to be saturated by lime). Additional water, called “spraying water”, is used to adjust the production rate and improve the mixing quality. Lime is brought in liquid form as milk of lime and is dissolved as it rises into the saturator, generating a fraction of calcium carbonates and other insoluble particles which can be continuously or discontinuously purged.

Conventionally, the dilution water used to dilute the milk of lime in the saturator is mineralised water or water that has been subjected to a remineralisation treatment. In the rest of this description, we will call this water “non-deionised water”, and it has simply been subjected to a so-called “non-deionising” filtration either by granular filtration on sand, or sometimes by micro-filtration or ultra-filtration, which has the disadvantage that it does not retain dissolved or undesirable molecules such as pesticides or nitrates. When included in the limewater designed to remineralise the deionised water, this water contributes to polluting it to a certain extent, admittedly not in large quantities but nevertheless undesirable. It is then possible to eliminate pesticides and/or nitrates from this water by a combination of means such as active carbon and biological denitration or denitration on resins, but the easiest way is often to use deionised water produced by the plant directly.

However, it has been observed that the use of deionised water that has either been thoroughly membrane filtered, or distilled, or water that has been subjected to a treatment with ion exchanging resins to produce lime water, instead of water on which only a non-deionising filtration has been carried out, led to the production of a lime water that quickly became cloudy due to precipitation of carbonates originating from atmospheric or dissolved CO₂. The turbidity of this type of limewater produced in this way from water that is very little or not at all mineralised can conventionally be between 50 and 100 NTU. This type of turbidity prevents its use during the remineralisation step of water on which a thorough membrane filtration has been carried out.

One purpose of this invention is to propose a new method for production of limewater from deionised water leading to a limewater from which all undesirable pesticide or nitrate type molecules have been removed, and in general from which all molecules not eliminated by non-deionising filtration have been removed.

Another purpose of this invention is to have lime saturators with efficiency greater than lime saturators supplied with filtered water. Saturator yields are conventionally between 75 and 95%, in other words the lime loss can be as high as 25%.

Another purpose of this invention is to present such a method to obtain limewater with a turbidity less than 10 NTU for a saturator efficiency greater than 95%.

Yet another purpose of this invention is to describe such a method that can be used in a lime saturator that is only slightly different from the lime saturator according to prior art.

These different objectives are achieved by the invention that relates to a method for production of lime water comprising a step in which milk of lime is mixed with dilution water, characterised in that the said dilution water is water deionised by membrane filtration or by distillation or by passage on an ion exchanger resin, and in that it includes a preliminary step consisting of adding silicate ions to the said deionised water.

Surprisingly, it has been observed that the use of silicate ions provides a means of obtaining a limewater with low turbidity and good saturation efficiency. Moreover, use in this way as dilution water for water that has been deionised, considerably reduces the probability of finding undesirable molecules in the water remineralised using limewater obtained using this method.

The use of such a compound can improve flocculation and precipitation, resulting in improved surface velocities greater than 2 m/h.

It should be noted that in the context of the method according to the invention, a silicate is used in preference chosen from the group composed of sodium silicate and potassium silicate, and that in preference, sodium silicate which has a lower treatment cost will be used.

As indicated above, the dilution water used in the context of the method according to the invention is not water that has been subjected to a coarse non-deionising filtration, but is water on which a thorough membranes filtration has been carried out or which has been distilled. This thorough membranes filtration may be a nano-filtration or a filtration by reverse osmosis.

Different means could be envisaged to add silicate ions to the deionised water. According to one variant, this addition step will be used mixing this compound. According to another variant, silicate will be introduced into the said saturator.

Furthermore, it should also be noted that silicate will be added to dilution water or into the saturator, preferably at a content of 5 mg/l to 40 mg/l, and preferably 10 to 20 mg/l, of SiO₂ with respect to the dilution water.

This invention is also intended to cover any installation for the production of limewater using the method described above comprising an installation for the production of lime water using the method described above, the said installation comprising:

-   -   at least one lime saturator,     -   means of bringing the dilution water into the said lime         saturator,     -   means of bringing milk of lime into the said lime saturator,     -   means of evacuating the limewater obtained by bringing the milk         of lime into contact with the dilution water,     -   means of evacuating unburnt materials and/or precipitates,

characterised in that it includes conveyance means for bringing silicate ions into the dilution water or into the saturator using a specific pipe.

Therefore the installation according to this invention is distinguished from classical installations in that it includes additional means of bringing silicate ions into the deionised dilution water.

Preferably, this installation also includes means of mixing silicate ions in the dilution water which, in the method according to the invention, is composed of ionised water on which a thorough membrane filtration has been carried out, or distillation, or treatment by passage on an ion exchanger resin.

Preferably, these mixing means capable of diluting silicate ions in dilution water include at least one mixing system which may be a static mixer or a mixing tank.

Finally, it should be noted that according to one variant of the invention, the installation also includes means of measuring the quality of limewater produced in the saturator and means of varying the silicate dose brought in by the conveyance means in the dilution water. Such measurement means may for example include a turbidimeter and/or a pH meter and/or conductivity measurement means.

This type of means can be used to add the silicate in just the right quantity to obtain the required quality of limewater produced using the installation.

The method and the installation according to this invention can be used to obtain lime water with turbidity less than 10 NTU with a saturation efficiency equal to at least 95%, that can be perfectly suitable for remineralising water on which a membrane filtration, or distillation, or resin treatment method has been carried out.

The invention also relates to any use of limewater obtained using the method described above in the context of a method to remineralise water to be remineralised by the addition of limewater and carbon dioxide.

The invention and its various advantages will be better understood after reading the following description of a non-limitative embodiment of the invention with reference to FIG. 1 that shows a diagrammatic view of an installation for the treatment of water on which a membrane filtration method has been carried out.

In FIG. 1, the installation comprises a lime saturator 1, means 2 of conveying the milk of lime into the said lime saturator 1, means 3 of conveying dilution water into the lime saturator, means 4 of eliminating the lime water obtained by bringing the milk of lime into contact with the dilution water in the lime saturator 1. It should be noted that conventionally, the lime saturator 1 also comprises a spraying water inlet 12 injecting water to facilitate mixing between the milk of lime and the dilution water. This lime saturator is also provided with a carbonates and insolubles trap 13 to evacuate carbonates formed in the lime saturator, and means 14 of evacuating sludge formed in the lower part of the saturator including an evacuation pipe 15 and a pump 16.

According to this invention, dilution water is composed of water that has been thoroughly membrane filtered, in this case nano-filtered water pumped by a pump 5 in a nano-filtered water reservoir 6.

Also according to this invention, the installation includes means 7 of bringing silicate ions into the dilution water. These means include a silicate reservoir 8, a pump 9 and a pipe 10.

Also according to this invention, the silicate ions are mixed with dilution water, for example using a static mixer 11, before this mix is added into the lime saturator 1. In the context of this embodiment, a static mixer 11 is used.

Also according to this invention, a turbidimeter 17 is provided on means 4 intended for evacuating limewater formed in the lime saturator 1, and means 18 of varying the silicate dose brought by means 7 for conveying this compound in the dilution water, are also provided.

This installation was tested using potassium silicate and sodium silicate. The concentration of silica, and the density and consistence of these reagents are shown in Table 1 below. TABLE 1 Concentration of Silicate SiO2 Density sodium 25 1.36 potassium 22.6 1.29

The total alkali strength (TAC) of lime water produced according to the invention was measured and compared with the TAC of lime water obtained conventionally, using water that has been subjected to a non-deionising filtration such as granular filtration on sand for production of lime water, rather than dilution water composed of water on which a membrane filtration has been carried out.

The coverage ratio of the TAC of limewater produced according to this invention, is measured and compared with the TAC of limewater obtained according to the state of the art. The turbidity at equilibrium of limewater obtained was also measured.

For potassium silicate, limewater production tests with additions of 0, 5, 6 and 20 mg/l of SiO₂ were carried out.

For sodium silicate, limewater production tests with additions of 0, 3, 9 and 32 mg/l of SiO₂ were carried out.

Tables 2 and 3 below show the results obtained for potassium silicate and sodium silicate respectively, with deionised dilution water. TABLE 2 TAC coverage ratio compared with TAC Potassium silicate obtained according content (in ppm of Turbidity at to prior art (in SiO₂) equilibrium (NTU) %) 0 >50 / 5 9 95-100 6 7 95 to 100 20 5 95 to 100

TABLE 3 TAC coverage ratio in comparison with Sodium silicate TAC obtained content (in ppm of Turbidity at according to prior SiO₂) equilibrium (NTU) art (in %) 0 >50 / 3 12 95-100 9 7 95 to 100 32 5 95 to 100

The addition of silicate according to this invention systematically causes precipitation of CaH₂SiO₄. Precipitates of this compound, in the form of 1 to 3 mm diameter flocs, settle at the bottom of the saturator, making the limewater clear. The quantity of flocs increases as the silicate content increases. This type of flocculation forms a complete substitute for cloudiness that can be caused by calcium carbonates when no silicate compound is added, in other words when membrane filtered water or distilled water is used for the production of limewater. The limewater between the flocs is perfectly clear when reagents according to the invention are added. When these compounds are not added, the limewater becomes cloudy and milky and its turbidity increases to about 50 NTU.

TAC measurements on the lime water produced from water on which a nano-filtration has been carried out gave results systematically between 210° F. and 230° F., and were always of the same order as results obtained on lime water produced according to the state of the art, from water filtered on sand. The calculated coverage ratio was thus always between 95% and 100%.

It should be noted that the embodiment of the invention described herein is not intended to reduce the scope of the invention. 

1-14. (canceled)
 15. A method of producing lime water comprising: a. deionising dilution water by membrane filtration, distillation or an ion exchange resin; b. adding silicate ions to the deionised water; and c. mixing milk of lime with the deionised water to form lime water.
 16. The method of claim 15 wherein said silicate ions are selected from the group consisting of sodium silicate and potassium silicate.
 17. The method of claim 16 wherein the silicate ions are sodium silicate.
 18. The method of claim 15 wherein the dilution water is deionised by membrane filtration and wherein the deionised water is subjected to a nano-filtration or a filtration by reverse osmosis.
 19. The method of claim 15 wherein the milk of lime and dilution water are mixed in a saturator and wherein the silicate ions are introduced into the saturator.
 20. The method of claim 15 wherein the silicate ions are added to the deionised dilution water such that the resulting mixture has a concentration of 5 mg/l to 40 mg/l of silicate ions.
 21. The method of claim 15 wherein the method is carried out in a system utilizing a saturator having a series of inlet lines and a silicate ion inlet line for directing silicate ions into the dilution water.
 22. The method and system of claim 21 including a mixer for mixing the silicate ions with the dilution water.
 23. The method and system of claim 22 wherein the mixer is a static mixer.
 24. The method and system of claim 21 including means for measuring the quality of lime water produced in the saturator and means to vary the silicate ion dosage.
 25. The method and system of claim 21 including a device for measuring the lime water quality where the device is taken from the group consisting of a turbidimeter, a pH meter, and a conductivity meter.
 26. The method of claim 15 including combining carbon dioxide and the lime water and using the combined carbon dioxide and lime water to remineralize water.
 27. The method of claim 15 wherein the addition of the silicate ions causes solids in the lime water to precipitate.
 28. The method of claim 27 wherein the addition of the silicate ions causes precipitation of CaH₂SiO₄.
 29. A system for producing lime water comprising: a saturator; an inlet for directing milk of lime into the saturator; an inlet for directing deionised dilution water into the saturator; a sludge outlet associated with the saturator; a deioniser for deionising the dilution water prior to the water being directed into the saturator; and a silicate ion inlet associated with the saturator for directing silicate ions into the saturator.
 30. The system of claim 29 wherein the deioniser comprises a nano-filter.
 31. The system of claim 29 wherein the deioniser comprises a membrane filter. 